1. Field of the Invention
The present invetion relates to a thermal transfer printer in which the major scanning is carried out by a thermal head and the minor scanning is carried out by the intermittent turning of the platen via a timing belt by means of a step motor, and more particularly to a thermal transfer printer which is adapted for reducing the noise that is generated by the intermittent turning of the platen.
2. Description of the Prior Art
As a thermal transfer printer there has generally been known a type in which one line is printed by pressing a heat-sensitive head, via an ink ribbon, against the recording paper which is wound on the platen, and the printing of the next line is carried out by intermittently turning the platen. An example of the prior art thermal transfer printer of the above kind will be described by referring to FIGS. 1 to 4.
As shown in FIGS. 1 and 2, the thermal transfer printer includes a step motor 10 which is arranged to turn a platen 14 intermittently through a timing pulley 12 and a timing belt 18 that is wound round a timing pulley 16 mounted on the platen 14. Here, the upper running portion of the timing belt 18 is set to be the tension side of the belt.
On the other hand, an ink ribbon 20 is arranged to be forwarded by a ribbon feeding motor 21 and a conveyor roller for ribbon winding 22 to an ink ribbon reel 24 by passing directly underneath the platen 14. With the ink ribbon 20 in between, a line-form thermal head 26 is arranged facing the platen 14.
Moreover, the recording paper 28 is fed from the paper supply cassette 30 by a paper supply roller 32, runs between the platen 14 and the ink ribbon 20 through guide plates 33 and a guide roller 35, and is printed on by the thermal head 26 after running around the platen 14 for about one half of its circumference. The recording paper 28 which has been printed is sent out to the paper removal tray 36 by a forwarding roller 34. The thermal transfer printer further includes a power supply unit 37 for driving the step motor and a controller 38 for controlling the turning and the like of the platen 14. The platen 14 is arranged to be supported by a frame 40 (see FIG. 3).
In the prior art thermal transfer printer with the construction as described in the above, the major scanning is carried out by the thermal head 26 while the minor scanning is carried out by the intermittent turning of the platen 14. Namely, the ink ribbon 20 is brought to a direct contact with the recording paper 28 which is wound on the platen 14, and the printing is accomplished by thermal transcription with the thermal head 26. In this operation, the thermal head 26 is pressed against the platen 14 with a force of several kg-weight so that there is required a large torque in order to revolve the platen 14 intermittently. In FIG. 4 there is shown, for the thermal transfer printer with the above construction, a force which acts on the platen 14 in its radial direction. In the figure, T1, T2, T3, T4, . . . , Tn are the tensions that act on the teeth G1, G2, G3, G4, . . . , Gn of the timing pulley 16 which is on the platen side. Here, it is known that there exists the following relationship among these tensions. ##EQU1##
In other words, the most significant is the tension on the tooth G1 so that it may approximately be set as ##EQU2## If the friction load vector due to the thermal head 26 is called F, the force acting in the radial direction of the platen is the radial force vector N which is the resultant of the tension vector T and the friction load vector F. In the prior art thermal transfer printer, the thermal head 26 is placed diametrically opposite to the tooth G1 of the timing pulley 16 on the platen side, with the shaft of the platen 42 as the center, so that the directions of the tension vector T and the friction vector F become approximately equal, producing a radial force vector N which is very large. The radial force vector N varies periodically due to the intermittent turning of the platen, which used to generate a noise when it is transmitted to the frame 40.
In order to prevent the transmission of vibrations like in the above and to suppress the generation of a noise, there has been tried in the past to insert cylindrical anti-vibration rubber pieces 46 between the frame 40 and the bearings 44 that support the shaft of the platen. However, such an attempt resulted in a new problem that the printing accuracy goes down due to a relative shift in the positions between the heat-sensitive head 26 and the platen 14.